I dont see how you could use a computer and NOT have more the one HD, or atleast a partitioned HD. One of the biggest reasons is for backup. If you have one HD and windows crashes or the HD fails and you cant boot, you will loose everything you ever had on your computer. EVERYTHING, and its not comming back unless you take it to a shop and they might be able to recover the data on it. But most people cant go without there computer that long, haha. Plus its a lot easier to just partition a drive or have a second drive. If you have any music or any important files on your computer you should have a second HD. Use one HD to install windows and games and programs and stuff that you have CD's for and can easily reinstall. Use a second HD for files that you want to keep like music files, stuff you download, pictures you dont wana lose, basicly anything you dont want to get rid of. Then when Windows crashes the on the main HD you can just reformat and install all the stuff you have CDs for and then everything on the other HD will just be there like nothing ever happened.
Like the saying 'once you go black, you never go back... Once you use 2 HDD's, youll never go back...